Flammable liquids such as gasoline, oil, and alcohol can be stored in floating roof storage tanks. Floating roof storage tanks are usually installed for environmental or economical reasons to limit product loss and reduce the emission of volatile organic compounds. Additionally, floating roof storage tanks sometimes are used to store flammable liquids because fumes of such materials can pose extreme risks of fire and/or explosion. Floating roof storage tanks therefore may be used to reduce the exposed surface area and reduce the vapor space of the flammable liquids, thereby reducing the risk of fires and/or explosions.
Lightning and/or other electrical discharges at or near the floating roof storage tanks, however, pose a serious risk of fire and/or explosion if the discharge ignites the fumes of the flammable liquids. This risk is particularly high at or near the rim of the floating roof of floating roof storage tanks, as an air gap typically exists between the floating roof and the shell of the floating roof storage tank. In fact, rim seal fires are the most common type of fire in floating roof storage tanks.
To mitigate the risk of rim seal fires, it is common to equip floating roof storage tanks with shunts around the rim of the floating roof. The shunts are spring loaded wiping contacts that engage the interior of the shell of the tank, thereby creating an electrical connection between the floating roof and the shell. The shell typically is connected to a grounding system, or may be considered inherently grounded because it physically rests on the earth, and therefore the floating roof can be grounded using shunts or other electrical connections between the floating roof and the shell. During a lightning strike, the lightning current passes from the floating roof, across the shunts, to the shell of the tank and into the earth. Thus, the threat of rim seal fires caused by electrical arcing or other electrical discharges between the floating roof and the shell can be reduced if the shunts and shells are maintained in working order. The shunts, however, tend to wear out over time and therefore may fail to reduce the risk of rim seal fires.
As an adjunct to the use of shunts, lengths of cable may be connected to the floating roof and to the shell of the tank. The length of the cable is dependent upon the distance which the floating roof moves up and down. The cables serve a similar function as the shunts in that during a lightning strike the lightning current is conducted from the floating roof, through the cables, and is dissipated through the shell of the tank down to the earth.
Today there are generally three types of floating roof storage tanks. One type of floating roof storage tank typically includes a cylindrical shell surrounding a floating top. The floating roof moves up and down within the interior of the shell but is exposed to the exterior environment directly overhead. There are also floating roof storage tanks commonly referred to as internal floating roof tanks (IFRTs). IFRTs also include a cylindrical shell but the shell includes a fixed top above the floating roof. The fixed top protects the floating roof from weather as well as defines a vapor space between the fixed top and the floating roof for collecting vapors seeping up from the interior sidewall of the shell and the floating roof.
The third type of floating roof storage tank also typically includes a cylindrical shell, but then rather than have a fixed flat roof, there is a dome on top of the shell. The dome is typically made of aluminum and therefore is commonly referred to as an aluminum dome roof tank (ADRT). The periphery of the dome rests on a lip on the sidewall of the cylindrical shell. Because the shell is made of materials other than aluminum, such as carbon steel, the shell and the dome expand at different rates due to temperature changes. An isolator bearing pad can be positioned between the periphery of the dome and the periphery of the shell along the inner lip to account for the variances caused by the differing expansion rates.
When flammable liquid is pumped through pipes from the storage tank, an electrostatic charge is generated that is commonly referred to as a “bound charge” because the static charge is bound to the flammable fluid. Therefore, small diameter, such as ⅛ inch, stainless steel cables may be installed within the interior of the IFRTs and ADRTs. These stainless steel cables facilitate the dissipation of the electrostatic charge by bonding the floating roof to the fixed roof of the IFTs or to the aluminum dome of the ADRTs, thus acting to equalize differences in electrical charge potentials. However, these stainless steel bonding cables are inadequate for conducting lightning energy because the lightning energy most likely will flow down the external surface of the dome and jump the insulated gap between the dome and the shell on the way down to the earth.
It is with respect to these and other considerations that the disclosure herein is presented.